Enormous resources and money have been committed to developing anti-skid control apparatus for automobiles and fail safe circuitry for the apparatus in recent years, and, as a result, some significant improvements have been made. According to the prior art, an anti-skid control apparatus is designed to work with the brake system of motor vehicles under the control of the operator. Typically a brake system includes a brake pedal, a hydraulic cylinder, a valve, an auxiliary cylinder and brake shoes operatively connected in series in a conventional manner so that when the operator applies a brake force by pressing the brake pedal, the force is transmitted through the hydraulic cylinder, the valve and thence to the auxiliary cylinder to force the brake shoes against the brake drum to slow down the vehicle. The vehicle tends to skid either when the road is slippery or when the brake is applied too forcefully so that the deceleration rate of the wheel is faster than the road surface condition would allow to roll without skidding. Typically, the anti-skid control system is designed to include means for sensing the deceleration or reduction in the rotational speed of the wheel and generate an `anti-skid` signal when the rate of deceleration exceeds a given rate, for example, 1 g. The "anti-skid" signal is then utilized by the anti-skid control apparatus for deactuating the valve so that the valve in turn deactuates the auxiliary cylinder and enables it to release the brake, and thereby allow the wheel to roll without braking.
The prior art anti-skid control apparatus includes a coil for actuating and deactuating the valve. The anti-skid control apparatus utilizes the "anti-skid" signal to energize the coil, and thereby operate the valve. Once energized, the coil causes the valve to release the auxiliary cylinder. In turn, the auxiliary cylinder releases the brake shoes and this causes the tire to rotate again without braking action. As the tire picks up speed again and the deceleration rate decreases below the predetermined minimum level, the "anti-skid" signal is removed from the anti-skid control circuitry. In turn, the anti-skid control circuitry removes the "anti-skid" control signal from the coil and thereby actuates the valve. At this point if the operator is still applying brake, the braking force from the brake pedal applied by the operator is allowed to go through the hydraulic cylinder and valve, and thence to the auxiliary cylinder, and allow the brake shoes to engage against the drum again. In this manner, the braking action is put under the control of the operator again.
In effect, the anti-skid control apparatus is designed to remove intermittently the valve control away from the operator when the deceleration rate of the wheel exceeds a predetermined level signifying skidding conditions. Thus the anti-skid control apparatus is designed to interrupt in a controlled manner the manual braking operation provided by the operator so as to minimize skidding of the vehicle.
Typically, the anti-skid control apparatus is made up of electrical circuitry which utilizes the power supply (i.e., battery) of the automobile and a voltage regulator which converts the output of the battery into a voltage level usable by the anti-skid control circuitry. In actuality, the battery voltage is very rarely stable and tends to vary significantly depending upon the age of the battery and the amount of usage. The batteries tend to vary from unit to unit in terms of their output voltage level. The voltage regulator is designed to generate a stable output for use by the anti-skid and other circuitry.
However, the output of the voltage regulator is found still subject to change in amplitude as the voltage amplitude of the power supply varies. The variation in the voltage amplitude of the output of the voltage regulator has been found to affect adversely the reliability of the performance of the anti-skid control circuitry of the anti-skid control apparatus. This renders the anti-skid control apparatus less reliable. Also it has been found that none of the anti-skid control circuitry have been provided with very reliable fail safe features for its fail-safe operation.